Modern optical communication systems, such as those used in the telecommunications field, typically use wavelength division multiplexing. Wavelength division multiplexing (WDM) is the sending of signals of different wavelengths simultaneously along the same transmission medium (normally an optical fibre). Each of the separate wavelengths can carry a separate information signal, thus allowing the simultaneous transmission of a number of different signals. Typically, each wavelength utilised to carry an information signal is referred to as a channel.
Information is carried within each channel by modulating the optical signal at a relatively high speed e.g. bit rates of 2.5 Gb/s to 40 Gb/s are common.
Analogue maintenance is the application of a relatively low frequency tone (less than the information bit rate), at a relatively small amplitude to the optical signal.
For instance, tones within the frequency range 300 kHz to 400 kHz may be used. Such a frequency is utilised to amplitude modulate the optical information signal at a relatively small modulation depth e.g. 1% of the peak power of the optical signal.
Analogue maintenance signals can be utilised to carry signalling data, for instance indicative of the source of the relevant channel, or for control of the optical network. For example, each channel within a WDM signal may be allocated a frequency band within the range 300 kHz–400 kHz, each band covering eight separate frequencies separated by 32 Hz. Signalling information can thus be transmitted by applying in turn different sequences of the eight frequencies to modulate the relevant optical signal.
As the analogue maintenance signal is applied at a predetermined proportion of the optical signal power, detection and determination of the amplitude of an analogue maintenance signal can be used to calculate the power in the relevant channel, thus easily allowing the monitoring of the performance of an optical link.
FIG. 1 shows an optical network 100, including a node 200. The node 200 includes apparatus for the detection of an analogue maintenance signal. In this instance, the node 200 also includes a receiver 300 for detecting at least one of the information signals carried by a relevant channel. The receiver 300 will typically include a WDM demultiplexer, arranged to demultiplex the optical signal into individual channels. Each individual channel can then be detected by a photo detector.
An optical signal, comprising a number of channels, is received from the rest of the network 100 by the node 200. The majority of this signal is passed to the receiver 300. A predetermined proportion of the signal (typically 1%–5%) is removed by optical tap 210, and passed to a photo detector 220, such as a photodiode. The photo detector converts the optical signal to an electrical signal. For convenience an amplifier 230 amplifies the electrical signal, the output voltage of the amplifier being passed to both a band pass filter 250 and a low pass filter 240.
The low pass filter 240 is arranged to pass only very low frequency components of the electrical signal (e.g. signals less than 1 kHz), with the resulting DC output voltage being indicative of the total power of the optical signal i.e. the sum of the powers within each channel.
The band pass filter 250 is arranged to filter out both the DC component of the input signal, and the high frequency components (e.g. those frequencies corresponding to the bit-rate of the information signal), and hence to pass those frequencies corresponding to the analogue maintenance signals. Typically, the band pass filter may be arranged to pass frequencies within the range 1 kHz to 1 MHz.
The signal passed by the band pass filter will thus contain frequency signals corresponding to each of the analogue maintenance signals for each channel. By measuring the relative amplitudes of each frequency component, the relative powers of each channel within the optical signal can be determined (as the power of a maintenance signal is a predetermined proportion of the total signal power). As the total power of the optical signal can be determined from the output of the low pass filter 240, consequently the optical power per channel can be determined. Further, by analysing the exact frequency present at any given instance, the signal information carried by the analogue maintenance signal can be determined.
Typically, to determine the relevant frequency components and amplitudes of the analogue maintenance signals, the output of the band pass filter 250 is passed to an analogue to digital converter 260, which digitizes the signal. The resulting digital signal is passed to a digital signal processor 270, which can determine both the frequency components and amplitudes of the frequency components present.
Stimulated Raman Scattering (SRS) results in the transfer of optical power between different wavelengths of optical signal being transmitted simultaneously along a transmission medium such as optical fibre. The power transferred between the different wavelengths is a function of the instantaneous channel powers.
FIG. 2 illustrates the Raman gain coefficients for a typical optical fibre in relation to a pump signal at 1530 nm. Within the graph, the solid line indicates the actual Raman gain coefficient, with the dotted line indicating an approximation of the Raman gain coefficient g that can be used to simplify the Raman power transfer calculation. As can be seen, the Raman gain coefficient within this range increases with increasing wavelength.
Consequently, an optical signal incorporating two or more channels at separate wavelength will see power being transferred from the lower wavelength channel to the higher wavelength channel as the signal is transmitted along the optical fibre.
It is an aim of embodiments of the present invention to provide apparatus and methods for improved optical processing of an optical signal, so as to improve the detection of the analogue maintenance signal.